Internal combustion engines exhaust a complex mixture of air pollutants. These air pollutants are composed of gaseous compounds such as nitrogen oxides (NOX), and solid particulate matter also known as soot. Due to increased environmental awareness, exhaust emission standards have become more stringent, and the amount of NOX and soot emitted to the atmosphere by an engine may be regulated depending on the type of engine, size of engine, and/or class of engine.
In order to ensure compliance with the environmental regulations, for instance with the regulations of NOX, exhaust gases may be treated by exhaust gas treatment devices. One strategy called selective catalytic reduction (SCR) for treating the exhaust gas may be implemented. SCR is a process where a gaseous or liquid reductant, for example, ammonia, ammonia water solution, urea or a urea solution, is injected into the exhaust gas stream of an engine. The reductant reacts with nitrogen oxides in the exhaust gas to form water and nitrogen. Usually, urea is introduced into the exhaust gases in an amount sufficient to provide the degree of NOX reduction desired. The desired amount of the reductant can be controlled by a urea injection system, for example.
Generally, SCR can be effective within a temperature range from about 200° C. to about 600° C. By providing a catalytic surface in the form of the SCR catalyst, the SCR process can be promoted and more efficiently performed, particularly at lower temperatures. Thus, it is important to provide a suitable temperature and a suitable catalytic material with a sufficiently large surface for the SCR reaction.
SCR as used herein generally includes those processes that utilize ammonia or any NOX reducing reagent capable of generating ammonia gas upon heating. The term “urea” as used herein is further meant to comprise all those reductants that are commercially available for performing SCR.
DE 10 2005 026 032 A1 discloses a method and device for treating exhaust gases. The device disclosed therein includes a particle separator, an SCR catalytic converter, and an ammonia generator for generating ammonia as a selective reducing agent for reducing nitrogen oxides. The particle separator is provided in a main exhaust line and the ammonia generator is provided in a secondary line.
U.S. Pat. No. 6,883,311 B2 discloses a compact dual leg NOX absorber catalyst device including a coaxial arranged dual leg apparatus comprising a housing having a first flow path and a second flow path having coaxially arranged portions. Furthermore, a device is disclosed for selectively directing the exhaust gases between the first flow path and the second flow path. Depending on a regeneration event of the NOX catalyst disposed within the first flow path, the device for selectively directing the exhaust gas may control the exhaust gas stream.
DE 10 2007 053 931 A1 discloses an exhaust gas treatment system and method for operating the same. The exhaust gas treatment systems includes a first flow path having a first catalyst and a second flow path having a second catalyst, wherein the first flow path is parallel arranged with respect to the second flow path and in a heat exchange relation.
The present disclosure is directed, at least in part, to improving or overcoming one or more aspects of prior systems.